The invention is related to Frequency Modulation (FM) detection using the so called quadrature detector approach. Specifically a monolithic silicon integrated circuit (IC) structure is employed to create an FM detector that is highly linearized.
Quadrature FM detection is well known in the IC art. A paper titled "applications of a Monolithic analog Multiplier" was published in the December, 1968 issue of the IEEE Journal of Solid-State Circuits (SC-3 No. 4) on pages 373-380. A commercial device using this sort of detection is identified as the LM1351 which has been available since about 1972. In this kind of FM detector a multiplier is driven from an IF limiter which also drives a single tuned circuit which in turn produces a quadrature signal that also drives the multiplier. The tuned circuit is adjusted so that the S-shaped detector transfer characteristic is centered at the IF.
Such detectors have proven to be very useful. Only a single tuning adjustment is required and the circuit is readily reduced to IC form which can incorporate other receiver elements as well. The associated FM receiver is quite straight forward and performs well. However, since only a single tuned circuit is used in the detector, the resultant transfer characteristic, while closely approximating a straight line near the center frequency tends to depart from a straight line when the FM signal deviates appreciably. Because of the wide deviation of a conventional FM signal the departure from a straight line introduces harmonic distortion, particularly for the louder signals. The distortion can be reduced by lowering the Q of the tuned circuit but this reduces the output signal level so that reduced signal-to-noise ratio occurs. It would be desirable to employ a tuned circuit Q that is high enough to maintain a high signal-to-noise ratio at the detector output and to avoid the harmonic distortion at wide deviation by other means. This would be of considerable value in the so called "High Fidelity" or Hi Fi reproduction of FM signals.